1. Field of the Invention
The present invention relates in general to an anti-lock braking system of diagonal type equipped with a proportioning valve, and more particularly to technical improvements of such a braking system that assure increased freedom or flexibility of braking pressure control while minimizing the number of pressure control valves used in the system.
2. Discussion of the Related Art
An anti-lock braking system (hereinafter abbreviated as "ABS", where appropriate) is a braking system capable of controlling braking pressures for individual wheels of a motor vehicle so as to prevent locking of the wheels during brake application. One example of a known anti-lock braking system is proposed in JP-A-3-169769, which is capable of controlling the braking pressures with a high degree of control freedom with a relatively small number of pressure control valves.
This known ABS, which is used for a four-wheel motor vehicle, is constructed with two mutually independent pressure application sub-systems connected to respective two pressurizing chambers of a master cylinder. The two pressure application sub-systems are connected to two front wheel brake cylinders and two rear wheel brake cylinders, in a so-called "diagonal" or X-crossing manner such that one of the pressure application sub-systems is connected to the front left wheel brake cylinder and the rear right wheel brake cylinder while the other pressure application sub-system is connected to the front right wheel brake cylinder and the rear left wheel brake cylinder. Described more specifically by reference to FIG. 14, each of the pressure application sub-systems (only one of which is shown in FIG. 14) has a front brake cylinder passage 204 which connects the appropriate one of the pressurizing chambers of a master cylinder 200 and a front wheel brake cylinder 202, and further has a rear brake cylinder passage 206 which is connected to the front brake cylinder passage 204 and to a rear wheel brake cylinder 208. Each pressure application sub-system is further provided with a first shut-off valve 210, a second shut-off valve 212, a reservoir 214, a reservoir passage 216, a third shut-off valve 218, a pump 220 and a pump passage 222.
The first shut-off valve 210 is a normally open valve disposed in a portion of the front brake cylinder passage 204 between the master cylinder 200 and a point of connection of the front and rear brake cylinder passages 204, 206.
The second shut-off valve 212 is a normally open valve disposed in the rear brake cylinder passage 206.
The reservoir passage 216 is connected to a portion of the rear brake cylinder passage 206 between the second shut-off valve 212 and the rear wheel brake cylinder 208. The reservoir passage 216 is connected to the reservoir 214.
The third shut-off valve 218 is a normally closed valve disposed in the reservoir passage 216.
The pump 220 is adapted to pressurize a working fluid pumped up from the reservoir 214.
The pump passage 222 in which the pump 220 is provided is connected at one end to the reservoir 214, and at the other end to a portion of the front brake cylinder passage 204 between the first shut-off valve 210 and the master cylinder 200.
The known ABS described above has a controller, which is connected to the first, second and third shut-off valves 210, 212, 218 to control these shut-off valves for controlling the fluid pressures in the wheel brake cylinders 202, 208 in a selected one of a plurality of pressure control modes, so as to prevent locking of the vehicle wheels. The pressure control modes include at least: (1) a first mode in which the second and third shut-off valves 212, 218 are held open while the first shut-off valve 210 is held closed, to reduce both the front wheel brake cylinder pressure (pressure in the front wheel brake cylinder 202) and the rear wheel brake cylinder pressure (pressure in the rear wheel brake cylinder 208); and (2) a second mode in which the first and third shut-off valves 210, 218 are held open while the second shut-off valve 212 is held closed, to increase the front wheel brake cylinder pressure with the pressure in the master cylinder 200 and reduce the rear wheel brake cylinder pressure.
The known ABS of the type indicated above is capable of adequate anti-lock control of the wheel brake cylinder pressures during brake application on an even friction-coefficient road surface and an uneven friction-coefficient road surface. The even friction-coefficient road surface is interpreted to mean a road surface whose friction coefficient is almost uniform over its entire area on which the four wheels of the vehicle lie. The uneven friction-coefficient road surface is interpreted to mean a road surface whose friction coefficient is different at the local areas on which the left wheels and the right wheels lie.
That is, there is a situation of brake application during vehicle running on an uneven friction-coefficient road surface, in which the front wheel whose brake cylinder is controlled by one of the two pressure application sub-systems of the braking system lies on an area of the road surface having a relatively high friction coefficient while the rear wheel whose brake cylinder is controlled by the same pressure application sub-system lies on another area of the road surface having a relatively low friction coefficient. When the above situation occurs, it is desirable that the front wheel brake cylinder pressure be controlled to be higher than the rear wheel brake cylinder pressure, for maximizing the braking force of the front wheel to utilize as much as possible the relatively high friction coefficient of the corresponding road surface area, while minimizing the amount of reduction of the lateral force of the rear wheel tire to thereby increase the steering stability or directional stability of the vehicle. The known ABS described above is not only capable of controlling the brake cylinder pressures in the second mode for reducing only the rear wheel brake cylinder pressure without reducing the front wheel brake cylinder pressure, but also capable of increasing only the front wheel brake cylinder pressure without increasing the rear wheel brake cylinder pressure. Therefore, the known ABS is capable of applying brake to the vehicle with a reduced braking distance without a loss of control of the running direction, even when the brake application takes place on the uneven friction-coefficient road surface.
If the braking system has a pressure increasing valve and a pressure reducing valve for each of the four wheels, the brake cylinder pressures of the four wheels can be regulated independently of each other. In this case, however, the braking system requires a total of as many as eight pressure control valves, two valves for each of the four wheels. The known ABS explained above uses a total of six pressure control valves, namely, three shut-off valves for each of the two pressure application sub-systems, which permit a higher degree of control freedom or flexibility of the braking pressures than in a braking system in which the braking pressures for all the four wheels are controlled in the same manner, although it is impossible to control the braking pressures of the two wheels in each pressure application sub-system completely independently of each other. Thus, the known ABS assures a relatively high degree of control freedom of the braking pressure with a relatively small number of pressure control valves.
However, further experiment and research made by the assignee of the present invention revealed the following problem with the known ABS described above.
The known ABS is a fluid recirculating type in which the brake fluid discharged from the wheel brake cylinders into the reservoir is returned by the pump to the master cylinder or a suitable fluid passage or other portion of the braking system in which the pressure is kept equal to the master cylinder pressure. In this fluid recirculating type braking system, the fluid pressure generated by the pump must be higher than the master cylinder pressure for the fluid to be returned from the reservoir toward the master cylinder. As a result, the pressurized fluid delivered from the pump tends to have a relatively large pressure pulsation, which is undesirably transmitted to the vehicle operator's foot through the brake pedal, as well known in the art as "kickback phenomenon". The pressure pulsation also causes a problem of the vehicle body vibration due to vibration of components of the braking system, and a problem that the mechanism including the pump and a motor for driving the pump and used for returning the fluid from the reservoir toward the master cylinder must have a large capacity, which makes it difficult to fulfill a need of reducing the size, weight and cost of manufacture of the braking system.
In the light of the above problems of the conventional ABS shown in FIG. 14, the assignee of the present invention developed improved anti-lock braking systems as shown in FIGS. 15 and 16, which are arranged to reduce the pressure pulsation of the brake fluid and incorporate a proportioning valve.
In the improved ABS of FIG. 15, the pump passage 222 is connected at its delivery output end to a portion of the front brake cylinder passage 204 between the first shut-off valve 210 and the front wheel brake cylinder 202. In this arrangement, therefore, the fluid pressurized by the pump 220 can be returned to the front brake cylinder passage 204 when the pressure of the fluid in the pump passage 222 is higher than the pressure in the front wheel brake cylinder 202, which is not always equal to the pressure in the master cylinder 200. Thus, the present arrangement permits easy reduction of the pressure pulsation upon fluid recirculation from the pump 220 to the front brake cylinder passage 204.
In the ABS of FIG. 15, a proportioning valve 230 is connected to a portion of the rear brake cylinder passage 206 between the rear wheel brake cylinder 208 and a point of connection of the passage 206 to the reservoir passage 216.
The ABS of FIG. 15 has a plurality of pressure control modes which include at least: (1) a first mode in which the second and third shut-off valves 212, 218 are held open while the first shut-off valve 210 is held closed, to reduce both the front wheel brake cylinder pressure and the rear wheel brake cylinder pressure; and (2) a second mode in which the first and second shut-off valves 210, 212 are held closed while the third shut-off valve 218 is held open, to increase the front wheel brake cylinder pressure by operation of the pump 220 and reduce the rear wheel brake cylinder pressure.
Consequently, the ABS of FIG. 15 is also capable of controlling the brake cylinder pressures in the second mode for reducing the rear wheel brake cylinder pressure without reducing the front wheel brake cylinder pressure, and increasing the front wheel brake cylinder pressure without increasing the rear wheel brake cylinder pressure. Thus, this braking system permits adequate anti-lock control of the brake cylinder pressures on not only the even friction-coefficient road surface but also the uneven friction-coefficient road surface.
On the other hand, the improved ABS of FIG. 16 differs from the ABS of FIG. 15 in that the pump passage 222 of the former ABS is connected at its output end to a portion of the rear brake cylinder passage 206 between the second shut-off valve 212 and the proportioning valve 230. In the other aspects, the ABS of FIG. 16 is identical with the ABS of FIG. 15. Namely, the arrangements of FIGS. 15 and 16 have a common design concept that the delivery or output end of the pump passage 222 is located downstream of the first shut-off valve 210. Therefore, the improved ABS of FIG. 16 also permits easy reduction of the pressure pulsation associated with the fluid recirculation by the pump 220.
It will be understood from the above explanation that both of the improved ABS arrangements of FIGS. 15 and 16 are adapted to reduce the required delivery pressure of the pump 220 for thereby reducing the fluid pressure pulsation by the pump 220. However, a further study by the present inventors found out some problems with these improved ABS arrangements as described below.
In the arrangements of FIGS. 15 and 16, the brake fluid delivered by the pump 220 is returned to a portion of the braking system which is upstream of the proportioning valve 230 as viewed in the direction from the master cylinder 200 toward the wheel brake cylinders 202, 208. Hence, the pressure in the rear wheel brake cylinder 208 located on the downstream side of the proportioning valve 230 is influenced by a pressure reducing effect of the proportioning valve 230 even while the braking system is in the anti-lock control mode, whereby the rear brake cylinder pressure cannot be made sufficiently high. Thus, the improved arrangements suffer from a problem that the friction coefficient of the road surface cannot be sufficiently utilized by the rear wheel to effectively brake the vehicle.
Generally, a proportioning valve is provided to establish an ideal or optimum distribution of the total braking force on the front wheels and the rear wheels of the vehicle. To determine the ideal braking force distribution, a shifting of the vehicle load in the vehicle running direction upon braking is taken into account. That is, when the vehicle is running forward, the load acting on the front wheels increases while that acting on the rear wheels decreases. A line A (hereinafter referred to "ideal distribution line") in FIG. 2 represents the ideal distribution of the braking force-on the front and rear wheels, which is determined by the characteristics of the proportioning valve. The ideal distribution line can or cannot be changed or adjusted depending upon the type of the proportioning valve. Where the ideal distribution line cannot be changed, this line is determined to achieve ideal distribution of the braking force during a vehicle run with a minimum load (hereinafter referred to as "minimum-load vehicle run"), namely, when the vehicle is running with only the driver. During the minimum-load vehicle run, the amount of reduction of the load acting on the rear wheels due to the load shift upon brake application is maximum. If the ideal distribution line of the proportioning valve is determined for ideal braking force distribution during the minimum-load vehicle run, the rear wheels cannot be effectively utilized for effective braking of the vehicle during a vehicle run with a maximum or full-load (hereinafter referred to as "full-load vehicle run"), namely, when the vehicle is running with the nominal number of passengers (including the driver). During the full-load vehicle run, the amount of reduction of the load acting on the rear wheels is smaller than that during the minimum-load vehicle run, if the vehicle deceleration by brake application is the same. This means that a larger braking force can be applied to the rear wheels without locking in the case of the full-load vehicle run than in the case of the minimum-load vehicle run. See lines C and D in FIG. 2. However, the proportioning valve designed to establish the ideal braking force distribution in the case of the minimum-load vehicle run cannot increase the rear wheel brake cylinder pressures for effective utilization of the rear wheels for effective vehicle braking in the case of the full-load vehicle run, while the rear wheel brake cylinder pressures can be increased without locking of the rear wheels during the full-load vehicle run.